This invention relates to a process for preparing biomedical devices. More particularly, it relates to such a process for preparing devices exhibiting piezoelectric properties by electrostatically spinning certain fluorinated polymers.
Medical devices, and in particular vascular grafts, have been prepared from polymer compositions using electrostatic spinning techniques. These techniques are well known and described, for example, in U.S. Pat. Nos. 4,043,331; 4,552,707; 4,798,607 and 4,657,793; U.K. Patent Application GB 2,181,207, and European Patent Application Nos. 0 223 374 and 0 239 339. The polymers which can be spun include polyurethanes and fluorinated hydrocarbons such as polytetrafluoroethylene (PTFE) . These polymers can be spun to prepare tubular structures suitable for vascular grafts, as well as fibrous mats or sheets suitable for use in wound dressings.
The polymers electrostatically spun to-date to prepare biomedical devices do not exhibit piezoelectric properties. A polymer is piezoelectric if it can convert mechanical energy or pressure into electrical energy, or vice-versa. In physical terms, piezoelectricity has been defined as the electrical polarization or charge produced by a mechanical strain in certain materials.
Through the use of selected polymeric materials, medical devices exhibiting piezoelectric properties and prepared by the electrostatic spinning technique are expected to display good biocompatibility, nonthrombogenicity, controlled porosity, oxygen and vapor transmission, and outstanding physical properties. In the body, the "piezoelectric effect" will be generated repeatedly by the effect of bodily movement upon the device. For example, when the device is a vascular graft, the regular beating of the heart and the pulsation of the blood vessels will act to provide the necessary mechanical strain on the device to induce the piezoelectric effect.
Certain piezoelectric polymers have been known for some time. For example, polyvinylidene fluoride and certain copolymers derived from vinylidene fluoride are piezoelectric. See Kawai H.:"The Piezoelectricity of Polyvinylidene Fluoride", Jpn. J. Appl. Phys. *:975, 1969, and Lovinger A.: "Ferroelectric Polymers", Science, Vol. 220, No. 4602, 1983. These piezoelectric polymers have found wide interest and acceptance in electronics, computer, avionics and audio and visual applications. Piezoelectric devices for these applications have been prepared conventionally as follows: a) the polymer is melt extruded to prepare a film, b) the melt extruded polymeric film is then subjected to a series of mechanical stresses or forces to induce an all-trans molecular conformation, and c) the stretched film is subjected to high voltage, plasma or corona poling to align the dipoles of each chain of the polymer in the direction of the field and induce an electrostatic charge. Steps b) and c) of this process are often referred to as "sequential stretching and poling".
While polymeric devices exhibiting piezoelectricity have found use for numerous applications, such devices have not been disclosed for biomedical applications. Furthermore, the conventional process for preparing piezoelectric devices, including stretching and poling of the device, limits the device to very specific geometries and forms, i.e., film or fiber, and is cumbersome, expensive and time-consuming. It would be extremely desirable to fabricate biomedical devices of various geometrical forms and exhibiting piezoelectric properties using a relatively simple and straightforward process.